Chapter 4 - Surface Analysis
Alfred G. Hopkins, Texas Instruments Incorporated,
Attleboro, MA 02703-0964
Since corrosion is essentially a surface phenomenon, those interested in the fundamental processes of corrosion have always been among the first to explore the utility of surface analysis techniques. These techniques have, and will continue to have, great success in illuminating many facets of corrosion phenomena.
General Principles
Surface analysis techniques generally require the sample to be placed in an ultrahigh vacuum (on the order of 10-7 Pa/10 -9 torr) to prevent contamination from residual gases in the analysis chamber. A rule of thumb is that up to an atomic layer per second can be formed at pressures of 10-4 Pa (10-6 torr) if each collision of a gas molecule results in its sticking to the surface. Fortunately, the sticking coefficients are often much less than unity, especially for oxidized samples such as often are of interest in corrosion studies. Some samples are especially prone to reacting with residual gases; this is often accelerated by electron beam-induced degradation at the sample surface with techniques such as Auger. In these cases, it may be necessary to bake the analysis system to reach high vacuums. With some techniques (such as Secondary Ion Mass Spectroscopy), the vacuum requirements can be relaxed since a surface that is actually being analyzed will only be exposed to the vacuum for a very short period of time. Near surface techniques, in contrast to true surface techniques, do not require an ultrahigh vacuum. Secondary Electron Microscopy - Energy Dispersive X-ray Spectroscopy (SEM-EDS) is an example of a particularly useful near surface technique that will be discussed in a later section.
The surface is then illuminated (interrogated) with high-energy electrons, photons, or ions. Depending upon the illuminating species, any or all of these types of species will be generated from the ensuing collision. Elemental identification is then made based upon characteristic energies or masses of the ejected species. One common way of characterizing surface analysis techniques is by tabulating the incoming and outgoing particles. Since surface analysis is an extremely specialized field, it has its own nomenclature; the reader is referred to ASTM E 673, Terminology Relating to Surface Analysis.
Surface analysis is mainly used in two separate modes. One is in surface science where the goal is to fundamentally understand the root causes and mechanisms that are occurring in a system. Usually a model system is picked to eliminate as many confounding variables as possible in order to get a system about which firm conclusions can be drawn. Often, many different techniques will be used on the same problem in order to illuminate as many facets as possible of the problem. The other mode is failure analysis. The goal here is to determine which of the failure modes (previously discovered by surface science) is the most important one for a particular failure. The samples are real and, hence, nonideal. This analysis mode is often used to identify the elements present, their distribution pattern, and their oxidation state.
The first and second most common of the true surface analysis techniques are Auger Electron Spectroscopy (AES) and X-ray Photoelectron Spectroscopy. They are electron spectroscopies that quantify the energy of electrons that are emitted by the surface during analysis.
Auger Electron Spectroscopy (AES)
AES is the most commonly used surface technique on metal samples
because of its following advantages [8]:
High surface sensitivity
Acceptable detectability for many corrosion problems
Simultaneous detection of all elements (except hydrogen and helium)
Very good small area analysis (mapping)
Ability to probe deeper into the surface by sputter profiling
Analysis time not excessively long
Readily available instrumentation.
The Auger process gives electrons of characteristic energy for each element, which are determined by the differences in energy of the orbitals involved. In addition to the Auger electrons, there are also much more plentiful secondary electrons with a broad energy distribution which overlay the characteristic peaks. To highlight the characteristic peaks, differentiation is performed on a plot of the number of electrons emitted by the sample versus the energy of those electrons. This results in a spectrum that ignores the more plentiful background (secondary) electrons and emphasizes the characteristic electrons which are used to identify the elements present. In some cases, the exact peak shape and energy can be used to identify the oxidation state of the elements present. Earlier generations of AES spectrometers collected the data in a derivatized mode, whereas today they collect the spectra in an integral mode and derivatize later.
One of the attractions of Auger analysis is that it is quite surface-sensitive since an Auger spectrum typically represents information about the composition of the top (0.5 to 2 nm) of the surface depending upon the sample analyzed and the analysis conditions. Although Auger electrons can be generated at depths of several micrometres into the sample, the Auger electrons must be able to escape to the surface without undergoing an inelastic collision in order to be detected.
Compilations of elemental spectra and charts of atomic number versus electron energy are available to help assign peaks. Modern data processing (background subtraction, peak fitting to standard spectra) has made it possible to correctly resolve many peak identification problems caused by peak overlap. The analyst should be familiar with ASTM E 827, Practice for Elemental Identification by Auger Electron Spectroscopy.
More information can be found in ASTM E 984, Guide for Identifying Chemical Effects and Matrix Effects in Auger Electron Spectroscopy
X-Ray Photoelectron Spectroscopy (XPS)
The realization that the energy of the ejected photon could be used to determine the chemical state of an atom caused the name ESCA (Electron Spectroscopy for Chemical Analysis) to be used. Because X-ray photons are necessary to generate appropriate electrons, the technique is also called X-ray Photoelectron Spectroscopy (XPS). XPS shares the Auger characteristics of good surface sensitivity since this is driven by the same need for the electrons to be able to reach the detector unscathed. It is possible to vary the depth of analysis in both techniques by varying the tilt angle with regard to the detector. This technique is used more extensively in XPS where it is often called angle resolved depth profiling. XPS also has the very important advantage of being able to obtain chemical state information on most atoms. A useful reference is ASTM E 1523, Guide to Charge Control and Charge Referencing Techniques in X-Ray Photoelectron Spectroscopy.
Secondary Ion Mass Spectroscopy (SIMS)
Secondary Ion Mass Spectroscopy (SIMS) is the third of the three most common surface analysis techniques. In SIMS, the sample is irradiated with a primary ion beam (normally argon), the impact of which sputters away the surface atoms, some as neutrals and others as ions. Those atoms which become ionized are then detected in a mass spectrometer where their masses are measured.
There are actually two variants of SIMS. The more common is Dynamic SIMS in which a high-energy ion beam is used that removes layers of the surface. The beam is so energetic that little chemical information is retained since the vast majority of any molecular species is fragmented. Although destroying the surface obviously prevents its re-examination, it is not a total disadvantage since it allows depth profiling to occur naturally.
Some of the advantages of SIMS are that it has a very low detection limit (PPM to PPT) and it can detect all elements. These advantages make it able to address many problems that neither AES nor XPS are suitable for. Reasonably small (micrometer or smaller) spot sizes allow elemental mapping. A major disadvantage of SIMS is that there is a very great range of ionization rates for different elements. Furthermore, the rates will vary depending upon the other species present (matrix effects). Either a beam of positive or negative ions can be used as the exciting beam and the response factors are much different between them. The biggest differences are found with the very electronegative halogens and the electropositive alkali metals. The variability in response factors makes quantification very difficult and closely matched standards critical. Of use is ASTM E 1505, Guide for Determining SIMS Relative Sensitivity Factors from Ion Implanted External Standards.
Near Surface Techniques
Although not a true surface technique, SEM-EDS often provides useful information in regard to surface corrosion mechanism. The ubiquitous nature, low cost, and ease of use of this technique cause it to be used as a tool in many failure analyses involving corrosion. Because its analysis depth is much larger (approximately a micrometre) than the true surface techniques, it is not necessary to analyze samples that are high-vacuum compatible. This results in the necessity of almost no sample preparation for many different kinds of samples.
The sample is scanned with a high energy (typically 5 to 30 KeV) electron beam in a raster pattern which causes the ejection of a number of particles including secondary electrons, backscattered electrons, and X-rays. Secondary electrons (with energies less than 50 eV) are only detectable if they are generated in the top surface of a sample; this causes the secondary electron output to be responsive to topographical detail and therefore gives an image that is remarkably similar to that seen with an optical microscope. Added advantages are greater magnification and depth of field. The contrast in backscattered electron images is mainly dependent on atomic number, so these images provide rough elemental distribution information.
Element identification is provided by analysis of the characteristic
X-rays that are emitted with an Energy Dispersive Spectrometer
(EDS). Quantification can be quite good if appropriate standards
are used. The X-ray detector can be set to only detect and count
X-rays that have energies within a narrow range. This output can
then be used to generate elemental distribution maps, or line
scans. Newer detectors with ultrathin windows can easily detect
all elements with an atomic number of 5 (boron) or greater. Some
applications of SEM-EDS analysis are given in the metallography
chapter of this manual.